In systems where a fluid under pressure flows from one point to another, it is frequently desirable to include a device for indicating the difference in fluid pressure at two discrete points. Such differential pressure sensing devices are widely used in industry and in the filter industry in particular. These devices provide a means for indicating the impedence of the flow of fluid through the system and can, for example, sense differential pressures across a pneumatic or hydraulic filter element such as occur when the element becomes clogged, and thereupon indicate that the filter element may require replacement or cleaning. Thus, it may be desirable to install such a device to monitor this difference in pressure and to provide an indication of when the difference exceeds some predetermined unacceptably high value corresponding to an unacceptably clogged condition of the filter.
Devices for providing such an indication are well known in the art, as described, for example, in U.S. Pat. Nos. 2,942,572, 3,140,690 and 3,815,542. While the devices described in these patents have proven reliable and effective, they are limited to indicating a pressure difference greater than a single predetermined value.
In many situations, indications of more than one predetermined pressure differential are highly desirable. For example, in the previously mentioned filtering system, it may be important to distinguish between the situation in which the filter is only partially clogged, although sufficiently blocked to require replacement during the next routine maintenance examination, and the situation in which the filter is completely clogged and requires immediate replacement on an emergency basis. This would require a first indication when the difference between the pressures at the upstream and downstream sides of the filter reaches a first level corresponding to the partially clogged condition, and a second indication when the difference between the pressures reaches a second higher level corresponding to the totally clogged condition.